This invention relates to thin film solar cells and more particularly to such solar cells having a layer of free metal between the transparent conductive layer and the semiconductor layer.
The art of thin film solar cells is well-known and improvements are being made therein in an effort to make such devices commercially practical. For example, amorphous silicon p-i-n structures are disclosed in U.S. Pat. Nos. 4,385,200 and 4,388,482 issued to Hamakawa on May 24, 1983 and June 14, 1983, respectively. Another Hamakawa et al patent, U.S. Pat. No. 4,410,559 issued Oct. 18, 1983, provides more detailed teachings concerning the glow discharge techniques for depositing amorphous silicon films with various doping materials. Each of these patents is hereby incorporated by reference for their teachings of cell structures and methods of manufacture.
While the amorphous silicon solar cells seem quite simple, the continued development has identified numerous limiting factors in such cells. The above-referenced patents discuss various ways of improving overall cell efficiency. As indicated in those patents, a somewhat typical solar cell comprises a transparent substrate such as glass supporting a transparent conductor layer on which is deposited the amorphous silicon active device on which is deposited a back face, usually metallic, conductor. Current is carried from the device by means of the front transparent conductor and the back metallic conductor. It has long been recognized that improvements in conductor resistivity, particularly the transparent conductor, can reduce device internal resistance and thereby improve efficiency. The ability to maintain peak current output near the maximum operating voltage is commonly referred to as the cell fill factor, CFF, with the higher rating being preferable. Internal resistances of the various conducting layers and interfaces therebetween must be reduced to improve the CFF.